A man armed with a knife has attacked passengers on a crowded bus in the northern German city of Luebeck before being overpowered and arrested.

Nine people were reported injured, one seriously.

Authorities had no immediate information on the assailant’s motive but they believe the suspect had no background in terrorism.

Prosecutor Ulla Hingst told n-tv television the bus driver stopped the vehicle and let the passengers out after the attack. She said the weapon used was apparently a knife but that is still under investigation.

Experts also examined a backpack on the bus as a precaution. Ms Hingst said it was not clear whether it belonged to the suspect but there had been reports of smoke coming out of it.

Police and prosecutors later said that investigators found a flammable substance in the backpack, but no explosives.

They said that the suspect was identified as a 34-year-old German citizen who lives in Luebeck. His name was not released, in line with German privacy rules.

“There are no indications that the man was politically radicalised,” the statement added. “There are also no indications at present of a terrorist background.”

Hans-Joachim Grote, left, an interior minister for Schleswig-Holstein, and Jan Lindenau, mayor of Luebeck, at the scene (AP)

Schleswig-Holstein’s state interior minister Hans-Joachim Grote said his information was that nine people were injured — six with the knife, two passengers who fell, and the driver, who was punched.

The statement from police and prosecutors said that one of the injured was seriously hurt, but said the total number of injured was unclear.

The bus was travelling from Luebeck to the neighbouring seaside resort of Travemuende, where an annual regatta, the Travemuender Woche, was opening later on Friday.

Mr Grote said that authorities believe security for the regatta is sufficient.